Wave-transmission system



Dec. s, 192s.

E. V. GRIGGS WAVE TRANSMISSION SYSTEM 4 Sheets-Sheet l Original Fileqlfeb. 1'7. 1922 -8. um E S MN :E mwuhw Zoom oomh. s NN 2 a; L Q w .n 2 2E om@ a mm1 com aoco. mmi @Si w QW .www mh E www@ E awww 232 -OSL-AML20Go. nQo- Room- ODE Illv Boom.

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E. V. GRIGGS WAVE TRANSMI S S I ON SYSTEM- oriigl Filed Feb. 17. 1922 4sheds-sheet 4 White Plains, in the county of YVestchester,

Patented Dec. 8', 1925.

UNITED` STATES PATENToF-FICE.

ELMER v. GRIGes'oF WHITE PLAINS, NEW YORK, AssIGNoR To WESTERN ELECTRICCOMPANY, INCORPORATED, OF NEWYORK, N. Y., A CORPORATION OF NEW YORK.

WAVE-TRANSMISSION SYSTEM.

Application mea February 17, 1922, serial Ne. 537,156. Renewed :une 2,was.A

T o all whom it may concern Be it known that I, ELMER V. Games, acitizen of the United States, residing at State of New York, haveinvented certain new andl useful Improvements in -Wave- TransmissionSystems, of which the following is a full, clear, concise, and exactdescription. -i' v l This invention'relates towave transmis-v sionsystems andmore'particularly to transmission of the energy ofgiven'waves or impulses by means of waves of'lower frequenc than thosewhose energy is to be transmitte An important eldot' electrictransmission is that involving the, use of unidirectional or alternatingcurrents'modified in certain of their characteristics, such ascontinuity, intensity or frequency. In this field, which includeselectric signaling and' other teledynamic electric operation, efficiencyfrom an ener standpoint is important but is frequengtl'y much lessimportant than faithful transmission of the wave forms involved.

" In tllecase'of telephony there is involved transmission'of a wide rangof essential frequencies and for certain circuits, such as those ofsubmarine cables, the distance to which such transmission may be had islimitedbecause of the dierence in transmission.-vl

ot the various frequency components involved. Various expediente havebeen re sorted to in an attempt to improve the trans; mission of suchcircuits for the higher fre-f quency components and to equalize theat-v` tenuation experienced by the different fre- V quen'cy components.These expediente in general involve change in the characteristics of thetransmission circuit itself or special terminal apparatus associatedtherewith. From a practical standpoint loading of the,-

circuit, particularly in the case'of submarine ponente aftertransmission to the same -p'ortional amplitude which they had beforecircuits is feasible only for limited distances. lnductance loadingwhich is commonly employed for such circuits also imposes anupperfrequeney limit'upon the transmission range o f the circuit lowerthan that possessed by the unloaded circuit. Attenuation equalizationmay be resorted to in lieu of loading in order Vto reduce each of thecomprotransmission. This requires elaborate teri' minal apparatus andhigh amplification and lis moreover subject to the difficulty that anydisturbance or interference is likewise highly magnified in theamplifying system. The

ymethod of the present invention attacks the art, such-r as loading andattenuation equal-` izers.

Since transmission of speech involves waves extending throughout a wideband of frequencies, it is necessary, if the intelligibility andAnaturalness ofthe speech are to be preserved to make'the transmissionsuch as to retain each of the initial frequency elements. `Where thehighest essential frequency elements exceed the upper -limit of thefrequency transmission range of the circuit, successful transmissionthereover is ,not possible. An Object ofthe present invention is toeffectively reduce the frequencies of the various components of a bandof currents so as to make them fall within thetransmission range ofcommercial transmission circuits. f Y

-' With this general view of the problem involved the principles kof theinvention may be outlined. Assume that a band of currents v rangingifroma definite lower frequency up 'to a fixed upper frequency is to betransmitted. According` to `the present invention there is subtractedfrom the frequency of each component of the-band. the frequency of thelowest frequency component. Thel band remains. 4of the same extent butits lower` frequency limit has become zero and.

its. upper frequency limit'has beenv so reduced that the bandmay fallwithin the transmission range of a circuit which could not successfullytransmit 'the original band. After transmission over` the circuit thefrequency of each com onent is augmented b v the original lower imitingfrequency and the components are thus restored to their originalfrequency position.

If the original band, `which it is desirable or necessary to transmit,is not continuous, but consists lof a number of smaller subbandsseparated by blank or unessential frequency intervals. the operationupon the lowest frel i quency subband will obviously be to subtract fromthe frequency of each ofits components the original frequency of itslowest frequency component. This operation reduces the first or lowestfrequency subband to a range from zero to a new upper limitingfrequency. There may be subtracted from the frequency of each componentofthe second or next highest subband a frequency sufficient to reduceits lowest frequency component to approximately the new upper limitingfrequency of the first subband. The third subband is similarly reducedin frequency so as to make its lowest frequency approximately that ofthe highest frequency of the reduced second band. This procedure is reeated with each subband in turn and the filial result is a continuousband from zero to a fixed upper limit. This continuous band may betransmitted and, after trans-v mission, maybe separated into its varioussubbands, and each subband may be stepped up in frequency to itsoriginal frequency position.

It may happen with the original band reduced in frequency in the mannerjust described toa continuous band extending up from` zero frequency,that its upper limiting frequency is still outside the frequencytransmission range of circuits commercially available. In this caseinstead of reducing the entire band to one continuous band, each of theoriginal subbands may be treatedindependently and reduced in frequencyby the original frequency of its own lowest component. yThere vwillresult a number of subbands each extending from zero frequency up andeach may be transmitted over an individual transmission channel orcircuit. The number of subbands and channels will be the same and. ifthis number is.

made suiiiciently'large the highest frequency of the original band maybe reduced to any desired degree. After transmission over their separatechannels each of the various subbandsmay be stepped up to its originalfrequency position, and the bands may then be reassembled -to reproduce.the original band.

The result of modulation of a carrier wave by waves of a band offrequencies is to produce two so-called side bands. See, for exam le,the article entitled Carrier current te ephony and telegraphy byColpitts and Blackwell, pages 307 to 310 inclusive, Journal of theAmerican Institute of Electrical Engineers, Volume 40, April 1921, No.4.-. Of these, the upper side band may be considered as the originalband of waves having' the frequency of each of its com` gonentsincreasedby the carrier frequency.

ince instead of a high carrier frequency there may be impressed upon themodulator a frequency changing wave of any definite frequency it isobviously possible to increase the effective frequencies of eachcomponent of the band by this definite amount. If the frequency ofthewave applied to the modulator is lower than that of the lowest componentof the original band, the lower or difference frequency Side -bandresulting from the modulating action may be considered as the originalband with its component frequencies, each reduced by the definitefrequency. There are accordingly available simple methods of stepping upthe frequencies of bands of waves or stepping them down at will.

The operation of both modulators and demodulators may be looked upon asthat of frequency changers. The modulator operates to change the freuency ,by the frequency of the so-called carrier wave applied thereto.The demodulator in general changes the frequency of a band of modulatedwave components by the frequency of the unmodulated carrier wavecomponent which is usually present with the side bands. In bothinstances upper and lower side bands are produced by the frequencychanging operation and it is necessary to select the desired side bandby means of filters or other selective circuits.

If in addition to producing the sum and Vdifference frequency componentsthe frequency changer were to transmit components of the original wavefrequencies as they occur before combination, there might be confusionif any components of the original waves fell within the same frequencyrange as the desired combination frequency components or side bands. Inorder to prevent this, use may be made of modulators and demodulators ofthe so-called balanced type disclosed and described in connection withFigs. 20 and 49 of the article Carrier cnrrent telephony and telegraphy.The characteristic of these balanced combining devices or frequencychanging devices is such thatl they may be -made to substantiallyprevent transmission of unchanged frequency components and to transmitonly components 1of the combination frequencies.

In order to insure that the frequenv)r increase 'at the receivingterminal may be the same as the frequency reduction at the transmittingterminal, it is desirable to derive the frequency changing waves at bothterminals from a common source which may be located at anyconvenientpoint in the system. Where several bands or snbbands undergo frequencychanges of dilhncnt amount it is convenient to derive the fr quencychanging waves from one base fri-- quency wave. by harmonic generatnm.Any well-known system of harmonic generation as, for example, thatillustrated and described in connection with Fig. i9 of the articleCarrier current telephony and telegraphy may be used.

The novel .features which are considered characteristic of the inventionare pointed llO .ilustrates a telephone system particularly adapted fora single loaded cable circuit; Figs. 2 and 3, a telephone system formultiple circuit submarine cable operation; Fig.

4', 'a detail of the system of Figs. 2 and 3;` and Fig. 5 an alternativeof this detail.

Referring to Fig. 1, an ordinary tele- -plione circuit 1 is shownconnected to al second telephone circuit 2 by means of a cable, loadedcircuit, or other toll link 3. The telephone circuit 1 is connected tothe cable 3 through a frequency changing apparatus at station A andcircuit 2 is connected to the cable by similar frequency changingapparatus atstation B. Speech currentsoriginating in or transmitted overcircuit 1 may comprise components from very low frequencies up tofrequencies of several thousand cycles per second. It is frequentlyassumed that the essential speech frequency lie within the range of 100to 2200 cycles. It is of course, possible to transmit intelligiblespeech by much more limited bands, lt is, moreover, not essential thatthe entire range be retained as portions of it may be omitted withoutserious loss. An improvement may be made vby transmitting` certaincurrents of higher frequenciesrepresenting such sounds as thesibilan-t's which are usually poorly transmitted, particularly whencircuits of limited frequency transmission range are employed,

Let it be assumed that good quality speech-I ti'ansinissionniay be hadAif the baiidsfrom 100 to 1000 cycles, 1500 to 2200 cycles. and@ 3600 toS800-cycles are retained and used.

Assume further that the cable or loa-dedcircuit 3 has a' transmissionrange lying `substantially entirely below 2150 cycles,- 'l`he functionof the apparatus at station' A is to reduce the frequencies ofthe bandschosen for good qualtiy transmission to .such

an extent that they may be faithfully,trans-fl initted by circuit Speechcurrents frenicii'cuit l are supplied by the hybrid coiljo'r' balancedtransformer 4 to outgoing channel 5 which includes three band filters 6,T, and 8, having their respective input' end sections connected inseries. These filters which may bc of the type described in ythe aiticleby Colpitts and Blackwell have transmission frequencv ranges, the limitsof which are indica-ted iii the. drawing. The

- filter 6 selects its band viz. of 100 to 1000 cycles inclusive.l andsupplies it directly to f and telegraphy,7 by Colpitts and Blackwell,

supplies oscillations of its base frequency, e.

`g., 50 cycles, through a low pass filterll to circuit 9. The harmonic'generator sup- Aplies harmonic oscillations of the 50 cycle basefrequency w-ave to an output circuit 40. Any other type of harmonicgenerator as, for example, that of Fig. 1 of vBritish Patent v131,426may be employed. Harmonic' oscillations of 500 cycle frequency areselected vfrom output circuit 40 by tuned circuits 12 and impressed ona, frequencyv the operation' of, the frequency changer 13 is to producea band of changed frequency in which -the original frequencies of theband passed by filter 7 are each 'diminished' by 500 cycles. Thisdiminished frequency bandis selected by band filter 14, the

transmission frequency limits of which-y range of the cable 3.

The currents transmitted to station B areA supplied by 'a hybrid coil 4to incoming circuitl'f includin 21 transmits only currents offrequency-well below 100 cycles, and according-ly selectsthe 50 cyclebase frequency ,current and supner similar to thaty described inthearticle by Colpittsandvlackwell. This harmonic l band filters 18,119 and20 and-low -pass fi ter 21 having respective .end sections Viiiseries..The low pass filter f changer or modulator 13 together with the Ilband, transmitted by filter 7. One result of 'plies it toharmonic.reprodueer 22 in a maiireproducer vmay be identical` in circuit'arrangement-with element 144` of Fig. 3. Repro neer 22 suppliesharmonic oscillations of various multiple frequei'icies to circuit 23.',Tuned circuits 24 select-1900 cycle oscillations and supply tliemtofrequency changer 25 to increase the frequencies of the band of'oscillations selected by filter 18 to their original magnitudes and theoriginal frequency subband of 3600 to 3800 cycles inclusive is thenseparated from resulting currents of other frequencies by filter 26 andsupplied to circuit 2. Similarly, 500 cycle oscillations are selectedand used to increase the frequencies of the band selected by filter 19to their original `frequency magnitudes. This band is then selectivelytransmitted by filter 27 to circuit 2. Filter 20 transmits theunchangedV band received fromt filter 6. Each of the various bands isthus restored to its original frequency position and together villvyield intelligible speech signals in a tele'- phone receiver associatedwith circuit 2.-

ATransmission. in .the lopposite direction, viz., from circuit 2 tocircuit- 1 is similarly accomplished. Filters 36, 37 and38 correspond tofilters 6, 7 and 8 'and serve toseparate out three subbands fortransmission the outgoin from the entire range of currents originatingin circuit 2. -The subband selected by filters 36 is sent direct to line3. That selected by filter 3 7 isreduced in frequency by 500 cyclesbefore transmission.l The third subband, that passed'byfilter 38, isreduced by 190D-cycles'. The 500 cycle and 1900 cycle currents used toreduce the frequencies of bands are selected from the output circu1t'23of harmonic reproducer 22 by tuned circuits. 'lhe operation at station Ain separating the various subbands received from station B and restoringeach to 'its Aoriginalfrequency position Will be obvlous from theforegoing explanation. The

- various harmonic frequency currents neces- Fig. 1, the characteristicof the transmission circuit 3 and the range of electric waves which areto be transmitted thereover are matched by operation upon the Waves tobe transmitted.v It may happen, as in the casel of submarine cables overwhich speech is to be transmitted`,that operation' upon the waves insuchmanner as'to retain the elements essential for -intelligibility at thedistant receiver will not reduce the frequency range to. that which canbe successfully transmitted by a single' submarine cable or othertransmission circuit. Figs. 2 and 3 together, illustrate a systemadapted to overcome this difficulty. A telephone line 101v terminatingat station X is shown connected with adistant telephone line 102terminating -at station Y by means of a system of cable lcn'cuits 103,104, 105, etc. By way of example, it may beassumed that these cablevcircuits have such transmission characteristics that each of them may beused to transmit currents of :tr-ange of.0 to 200 cycles frequency'without excessive distortion or prohibitive f attenuation. The numberof such cablecircuits-necessary' will be dependent upon-thistransmission range and will increase as the-practical transmission bandof each circuit decreases. The cable circuits may be metallic or may beground return and the drawing is intended to indicate diagrammaticallyeither type of circuit. Moreover, the conductorsof these circuits may beplaced in individual cables or may all be placed in the same cable,depending upon circumstances. The latter form of construction ispreferable from the. standpoint of economy in the first cost Wherever itis permissible from the standpoint of transmission requirements. Each ofthese cables may be of the periodically loaded (Pupin) or continuouslyloaded (Krarup) types. Attenuation equaliz'ers may also be employed tocounteract the effect of different line attenuations for differentfrequencycomponents by giving such attenuation to each component thatthe added attenuations caused by the. line and the equalizer are thesame for each component. Various attenuation equalizer circuits are Wellknown. Although any desirable type may be used, that preferred in thissystem` and illustrated, is of the type disclosed at Figs. 9 and 10 ofU. S. patent to Hoyt No. 1,453,980, dated May 1, 1923. Its advantage asan element of a system in which filters are employed is obvious. It isto be understood that the various receiving circuit-devices referred toas band filters, such as 18, 19, etc. in Fig. 1 and the correspondingfilters in the system of Figs. 2 and 3 comprise not only frequencyselectingA elements as in the case of the ordinary band filter, butAalso elements designed as set forth in the Hoyt patent to give anattenuation complemental to that of the main or toll transmission link.

Line 101 terminatesa't station X in a net N which simulates itsimpedance throughout the entire frequency range of the currentstransmitted thereover. A hybrid coil or balanced three windingtransformer, similar to element 4 of Fig. 1 serves to connect line 101to incoming and outgoing channels 10T and 108, respectively. Channel 108is provided Wit-h transformers 109, 110. 111, 112, etc.,.separatedrespectively by high pass iiiters 113, 114, 115, ete. Transformer 100asses outgoing currents from line 101 to a ow pass filter which selectsthose of frequency below 200 cycles and transmits them to line' 103.High pass filter 113`readily --transmits currents of all frequencieshigher than 200 cycles to transformer 110 which supplies them to a bandlilter having a transmlssion range extending from -200 to 400 cycles.This band filter impresses ils selected bandupon frequency changer 120which is of the same general type as the modulating,r elementillustrated at Fig. 20 of the article Carrier currenttelephony andtelegraphy.

A harmonic generator 1.30 of the type illustrated at Fig. 49 of thearticle Carrier current telephony and telegraphy comprising an electricdischarge oscillator having coupled input and output circuits and a highresistance element in its space current circuit produces base-frequencyoscillations of 100 llt) lil)

dit

device which serves cycles fre uency and harmonics thereof and suppliest em through a thermionic amplier also having a high resistanceelementin itsspace current circuit to circuit 131 from which tuned circuits 132select 200 cycle currents for use with frequency changer 120 in reducingthe frequencies of the outgoing band transmitted thereby. Frequencychanger decreases the frequency of the transmitted subband4 by 200cycles. This decreased frequencysubband is then selected by a bandfilter 133 having a transmission range-from 0 to 200 cycles and isimpressed upon outgoing line 104. Currents trans-v mitted over line 104are supplied at station Y to a frequency changer 140,l which in'-creases the frequency of the received subband by 200 cycles,"fand thisincreasedfrequency subband is then selected by ai band filter andimpressed upon incoming channel 151 by which it is transmitted 'to line102. The frequency changing operation of the device 140 is accomplishedby combining the received subband with 200 cycle b Waves'l supplied bytuned circuits 141 through an amplifier. Circuit 106 is used to transmit100 cycle base frequency current from station X to the remote stationrllhis current is selected b a lovv` pass filter 143 and is supplied toa liarmonic reproducer 144 of any 'desired type which supplies harmonicfrequency currents to its output circuit 145. 'lhe harmonic reproducercomprises three electric discharge devices in tandem. The first of thesereceives the base frequency current and amplifies it to such an extentthat the amplified base frequency Waves impressed upon the secondelectric discharge device cannot be faithfully repeated thereby. Thisproduces a distortion of the base frequency Waves as repeated in theoutput circuit of the second electric discharge device and a consequentgeneration of harmonics thereof. In order to increase the distortingaction of the second electric discharge device, it is provided with ahigh resistance in series with its space current The distorted basefrequency wave charge device is impressed upon the input circult of athree element electric discharge ics and supply them to circuit 145.

In a similar manner currents originating in line 102 are separated intovarious frequency subbands each of 200 cycles extent and each subband isreduced to the range cf 0 to 200 cycles before transmission to stationX. After reception at station X, it is again restored to its originalfrequency position. It is of course to be understood that the frequencyrange of the currents transmitted over each individual cable will bedetermined by the circumstances.y Moreoveigas `supplied to the inputcircuit 176 is "aSSllmE a various harmonics in the' output -clrcuit ofthe second electrlc d1sto amplify the harmon-v in the system of Fig. 1,certainportions of thedspeech frequency spectrum may-"be discar eCircuit 106 is associated at station with a. band pass -ilter whichtransmits currents of a narrow band of frequencies in- Tlus circuitserves there- .of the incoming subbands. The gain control device 200 atstation Y is associated with the input circuit 17 6 of the harmonicreproducer 144 so as to be controlled by the asel frequency wave of 100cyclesfrequency transmitted from station X. K f

Fig. 4 illustrates the operation of the gain control element tiaincontrol current of 100 cycles impressed] upon the grid-lament circuit ofa three.- element electric discharge device 191 which includes a seriesblocking condenser-177, a polarizing source 178 and a high resistancegrid leak 179. ri`he space current in the output circuit of the electricdischarge ,Y de vice will be determinedin the absence of gain controlcurrents largely by the polarization, potential impressed onk the gridby source 178. A solenoid having ak Winding 180 connected in the spacecurrentcircuit will cause spring retracted plunger 181 toA positiondetermined by the strength of this space current. v.As the gain'controlcurrent increases, the potential of the grid becomes more negative -inthe same proportion in accordance with the Well-known operation of thegrid lealv demodulators and theV space current falls. As a result ofthis action, the plunger 181 movesand carries a circuit closer 183 '.toa position in engagement with. a different -contact 184. The circuit ofthe circuit closerl may be traced from ground by way of'battery 182,circuit'- closer 183, one of the contacts 184, and a winding 4of thecorresponding relay 1851to ground.

The relay 185 uwhich energizes', lclosesits armature 186, thusdeterminingthe portion of the` potentiometer resistance 187 to beincluded in the input circuit of amplifier 188 `or 189, as the case vmaybe, 4and thus in the system of Figs. V2

to determine the amplification ofthe sub-` bands received-at station Y,v

Fig. '5 illustrates a modification of the 'arrangement of Fig. 4,involving no moving elements. Only so much of the circuits are shown asare necessary to explain theap-l plicatioii of this -modifiedarrangement to one for each of the transmission circuits' 103, 104, 105,etc.

Element 190 is substantially identical with electric discharge device191 of Fig. 4, except for the substitution of manner' that the aresistance network for the v'solenoid 180 and its associated elements.This network, comprises resistances 192 and 193, each `of very highmagnitude, as of the order of a half megolim, a resistance 194 Iin`shunt thereto and a capacity element 195 which serves as a lowimpedance for all alternating currents to divert them from theresistance paths. Space current-passes mainly by Way of resistance 194which maybe of a few thousand ohms to match the internal space currentresistance of the, device 190. Re-

sistance 193 is of such magnitude as to effectively separate the inputcircuit of ampliiier 188 and the output circuit of gain control element190 so far as alternating current is concerned. The only action ofdevice 190 will therefore be to impress a direct current E. M. F. acrossresistance 192. The resistance 192 is included in the polarizinggridcathode circuit of amplifier 188 in such a difference of potentialcaused by the flow of space `current therethrough. tendsy to oppose thepolarizing source 196. The result is that the grid potential ofamplifier 188- may be only slightlynegative or even positive and theamplifying action of the device 188 will be large. As the energyreceived over circuit 176 increases, the space current of device 190falls and likewise the potential difference across resistance 192.amplifier 188 therefore becomes more negative and the amplifying actionof thisfa-mplifier decreases, thus compensating for the increasedtransmission on circuit 103.

The transmission` of the base frequency current from stat-ion X tostation Y serves a vdouble function in that it enables the frequencychange at station Y to be made exactly theconverse of that of station Xand moreover, thatit may serve to determine the operation of thegaincontrol apparatus.

In the circuit arrangement of Figs. 2 and 3, the attenuationequalization o eration may be-combined with the select-ing unction ofthe receiving band filters such` as filter 150. The ensuingamplification must of course be sufficient to augment the attenuatedcurrent, but it should be borne in mind that if the frequency changer isof the electric discharge type illustrated, it introduces anamplification factor.

Although the invention'has been disclosed as embodied in certainspecific arran ements,

the principles of the invention are o )viously The grid of 'to reducethe component frequencies by a predetermined amount, transmitting saidwave of reduced frequency components and said single frequency wave to adistant station, and utilizing said single frequency wave to increasethe component frequencies to their original magnitudes.

2. The method of transmitting a broad band of Waves which comprisesdividing saidband into va plurality of subbands, reducing each of saidsubbands by the frequency of a harmonic of a given base frequencywave,'transmitting said reduced frequency subbands to a distant station,transmitting said base frequency wave to said distant station, derivingharmonics of said base frequency wave, and increasing the frequency ofeach of said subbands by combination with a wave of the same frequencyby which it was reduced before transmis- 100 sion.

8. The method of transmitting a broad band of waves which comprisesdividing said band into a plurality of subbands, combining each of saidsubbands'with a har- 105 inonic of the same base frequency wave toreduce the effective frequencies of said subbands, transmitting. saidreduced frequency subbands and energy of the base frequency wave toadistant station, deriving haimonies of said transmitted base frequencywave, and combining each of said transmitted subbands with one of saidharmonics to restore the subband to its original frequency position.

4. 'A system foi` transmitting a band of waves between twostationscomprising a source of Waves of a definite frequency, means forsupplying waves from saidsource to each of said stations, means forcombining waves derived from said source with said band atthe first ofsaid stations to reduce the effective frequency of said band, means fortransmitting said reduced, frequency band to said second station, andmeans at said secondstation for combining said received band'with wavesderived from the waves from said lsource to restore said reducedfrequency band to its original frequency position.

5. 'A system comprising two separated stations, a source of basefrequency currents at the first of said stations, means for supplyingbase frequency currents therefrom to the second station, means forproducing a broad band of waves at the first station, means for dividingsaid band into a plurality of non-overlapping frequency subbands, meansfor deriving harmonics of said base frequency Wave vand reducinfi theeffective frequency of each of said su bands by the frequency of one ofsaid harmonics, means for transmitting each of said reduced f requencysubbands to said second station, means at said second station forproducing harmonics of the base frequencywave transmitted thereto, andmeans for combining each of the transmitted subbands with one of saidharmonics at said second station to restore said subhand to its originalfrequency position.

6. Means for transmitting between two y stations a wave havingcomponents of a given range of -frequnecies which comprises means forreducing the frequency of each of said components by a predeterminedamount, means for transmitting said reduced frequency components fromone station to the other, means at said second station for increasingthe frequency1 of each component of said received wave, means fortransmitting between said stations a wave of a definite frequency, andmeans for causing said denite frequency wave to control the `frequencychange of said transmitted band.

7. A system for two-way transmission between distant points comprisingmeans for transmitting base frequency energy therebetween, means forcausing -said base frequency energy at each of said points to reduce theeffective frequencies of signal energy to be transmitted and to increasethe effective frequencies of signal energy received by like amounts.

8. A, continuously loaded cable circuit, means for transmitting currentsof a' plurality of frequencies within a range smaller than the effectivefrequency transmission range of saidcable but extending betweenfrequency limits at least one of which is higher than tlie upper limitof said effective transmission range, and a frequency changing meansconnecting said transmission means to said cable to reduce the effectivefrequencies of all of said currents to magnitudes within thel limit/slof said -effective transmission range.

In witness whereof, I hereunto subscribe my name this 10th day-ofFebruary A. D., 192

`ELMEVR v. caress.

